1. Field of the Invention
This invention relates to the bonding of wires to a substrate, and more particularly, to a method and apparatus enabling deep access, close proximity, fine pitch ultrasonic bonding of large wire.
2. Description of the Related Art
Wire bonding is the process of making electrical connections in semiconductor components by means of electrically conductive wire, typically wire with a diameter of from 12 microns to 500 microns. Examples of electrical connections which can be made using wire bonding techniques include connections between the contact surfaces of discrete or integrated chips and the contact leads of their packages, and, in the case of hybrid circuits, the connections between inserted monolithic elements and the film circuit which contains them.
A number of wire bonding techniques have been developed, and one which has been particularly successful is a microwelding technique using ultrasound. An automatic wire bonding apparatus on which such a technique can be operated is described in U.S. Pat. No. 4,619,397. Aluminum wire, for instance, in contact with the contact surface to which it is to be bonded, is moved vigorously along the surface to which it is to be bonded so that its oxide layer breaks open. The wire is then subjected to pressure, and a permanent junction is created between the two materials. Motion of the wire is generated by an ultrasonic transducer excited by an ultrasonic generator to produce high-frequency mechanical vibrations.
In the particular wire bonding process known as wedge bonding, the ultrasonic energy is supplied at a level correlated to the wire size used. The ultrasonic energy is directed to the aluminum or other type of wire by a special tool known as a "wedge." The wire is fed through a guide at the bottom of the wedge. When the wedge with the wire touches the surface to which the wire is to be bonded, movement is stopped. The wire is pressed down with a small defined force, known as the bonding weight, and the wire is slightly deformed. This small deformation is known as the "pre-deformation." Ultrasonic energy is now switched on, and the welding process starts. During this time, the diameter of the wire is reduced by a few microns, the actual reduction depending on the size, physical properties and the precise chemical nature of the wire.
Large wire bonding is defined as attaching about 0.005-0.020 inch (about 125 microns to 500 microns) electrical conductive wire to power devices as used in the semiconductor industry. Wires may be aluminum (Al), gold (Au) and copper (Cu), for instance, with aluminum wires being most prevalent. However, wires other than Al, Au and Cu may be used. Ultrasonic energy and elevated temperature of a work piece (in some cases) is the principle method of attaching these large wires to the power devices and/or semiconductors using a bonding tool or wedge.
Wire bonders, and especially large wire bonders, often require a cutter to terminate the wire. This cutter is typically positioned adjacent the bond wedge and brought down with a Z-axis motion to cut wire after a bond has been made. One example of a cutter positioned adjacent a bonding wedge is shown in U.S. Pat. No. 5,906,706.
Present wire bonders bring wire to the bonding tool, for example, by an independent wire guide of two types. In the first type, a separate member consisting of a wire guide capillary is held in place behind the wedge and at a specific angle to the bonding wedge. This wire guide capillary is disclosed, for instance, in U.S. Pat. No. 5,452,838. In the second type, a "clip on" wire guide member that separately attaches to the bonding wedge. The "clip on" wire guide member is disclosed, for instance, in U.S. Pat. No. 5,906,706.
In both wire guiding methods, the wire is held in position so as to locate the wire directly below the tip of the bond wedge. After the wire is brought to the work piece or substrate, loop forms are made with a combination of machine tool motion and friction within the two types of wire guides described above.
Unfortunately, the prior art tools require too much room to work with many emerging chips. As integrated circuits are made smaller, the resulting chips have less access space within which to bond wires. The combination of the bonding wedge or tool and separate wire guide described above precludes these wire bonders from being able to access newer power devices that call for deep-access, close proximity, and/or fine pitch wire bonding. Fine pitch wire bonding refers to wires bonded side-by-side as close as possible. Furthermore, when a wire cutter is used, the cutter adds additional dimension near the tip of the bonding tool making it even more difficult to access deep, tight spaces on a substrate.